The plasma membrane H(+)-ATPase of Saccharomyces cerevisiae is an essential protein that is required to establish cellular membrane potential and maintain a normal internal pH. An Asp-378 to Asn substitution at the residue phosphorylated during catalysis is dominant lethal when the pma1-D378N mutation is expressed along with a wild-type plasma membrane H(+)-ATPase (PMA1) gene. Several mutations in the first two putative transmembrane domains are also dominant lethal. However, these dominant lethal mutants often appear to be innocuous, because they are frequently lost by gene conversion to the wild-type sequence during the process of introducing the mutant sequence and subsequently removing the wild-type gene. Loss of the mutation by gene conversion does not occur while introducing recessive lethal mutations. Cells carrying the wild-type PMA1 gene on the chromosome and a dominant lethal mutation under the control of a GAL1 promoter on a centromere-containing plasmid exhibit a galactose-dependent lethality. Indirect immunofluorescence staining using anti-Pma1 antibodies shows that induction of dominant lethal PMA1 mutations leads to the accumulation of a number of intensely staining cytoplasmic structures that are not coincident with the nucleus and its immediately surrounding endoplasmic reticulum. These structures also accumulate the endoplasmic reticulum protein Kar2. Expression of the dominant lethal protein also prevents transport of the wild-type ATPase to the plasma membrane.